Greases are used in a variety of engineering applications to maintain a lubricant between moving machine surfaces, such as found, for example, in bearings. A grease exhibits shear-thinning rheological properties, which means that the viscosity of the grease reduces under shear. A grease typically comprises a lubricant, such as a lubricating mineral oil, and a thickener substance. Under non-shear conditions, the thickener acts as a sponge-like matrix that holds the lubricating oil. As a result, the grease exhibits the characteristics of a solid or semi-solid. The lubricating oil may be released from the matrix both under static or dynamic conditions. In particular, when a shear force is applied, the lubricating oil separates from the thickener matrix and results in the grease exhibiting lubricating properties.
The properties of the lubricating oil are important in determining the lubricating properties of the grease. In addition, the interaction of the lubricating oil and the thickener is important in determining, for example, the longevity of a grease when used as a lubricant. In order to prolong its life as a lubricant, the oil replenishing characteristics of the grease are critical to ensure that the lubricating oil is reabsorbed into the matrix once the shear is removed from the lubricated system.
Lubricant greases for bearings are described in EP 0700986, EP 0795597 and EP 0795598. In these documents, a polymer is used as the thickener in a grease composition. The lubricating oil is chosen to be a conventional synthetic lubricating oil, such as a mineral oil, a synthetic hydrocarbon oil or an ester oil.
As described in EP 0700986, a typical method for the manufacture of a grease is to form a homogeneous mixture of the desired lubricating oil and thickener, and then to rapidly cool or quench it. This may be achieved by pouring the composition onto a cooling table.
The present invention aims to address at least some of the problems associated with the prior art or at least to provide a commercially useful alternative thereto.